1. Field of the Invention
The present invention relates to an apparatus and method for controlling power levels of individual signal lights, or channels, of a wavelength division multiplexed (WDM) signal light travelling through an optical fiber. More particularly, the present invention relates to an apparatus and method for determining the spectrum of the WDM signal light and controlling the power levels of the individual signal lights in accordance with the determined spectrum.
2. Description of the Related Art
Generally, conventional optical communication systems connect a transmitting station to a receiving station through an optical transmission line. Moreover, wavelength division multiplexing (WDM) techniques can be employed to increase the transmission capacity of the optical communication system.
FIG. 1 is a diagram illustrating an optical communication system which employs WDM techniques. Referring now to FIG. 1, a transmitting station 2 produces a WDM signal, and provides the WDM signal to an optical fiber transmission line 4. The WDM signal travels through optical fiber transmission line 4 to be received by a receiving station 6.
A plurality of optical repeaters 10 are provided along optical fiber transmission line 4. Each optical repeater 10 includes an optical amplifier 8 for amplifying the WDM signal. Typically, optical amplifiers 8 are erbium doped fiber amplifiers (EDFA).
FIG. 2 is a diagram illustrating a conventional transmitting station 2, for use in the optical communication system illustrated in FIG. 1. Referring now to FIG. 2, transmitting station 2 includes m optical transmitters 12 (#1 to #m) corresponding, respectively, to a first channel (ch. 1) to an m-th channel (ch. m), where m is an integer greater than 1. Each optical transmitter 12 (#1 to #m) produces a signal light having a wavelength which is different than the wavelength of the signal lights produced by the other optical transmitters. More specifically, the wavelengths of the signal lights produced by optical transmitters 12 (#1 to #m) are set to .lambda.1 to .parallel.m, respectively.
Optical transmitters 12 (#1 to #m) include laser diodes 14 (#1 to #m), respectively, and optical modulators 16 (#1 to #m) respectively. Thus, laser diodes 14 (#1 to #m) generate carrier lights having predetermined wavelengths, and optical modulators 16 (#1 to #m) modulate the carrier lights with information. More specifically, for example, optical transmitter 12 (#1) includes laser diode 14 (#1) and optical modulator 16 (#1). Laser diode 14 (#1) generates carrier light at a wavelength .lambda.1. Optical modulator 16 (#1) modulates the carrier light with information from a main signal (not illustrated).
The signal lights from ch. 1 to ch. m are received by an optical multiplexer (MUX) 18 through optical connectors OC. Optical multiplexer 18 wavelength-division multiplexes the signal lights together, to form a wavelength division multiplexed (WDM) WDM signal light. The WDM signal light is provided to optical fiber transmission line 4 for transmission to other devices (such as receiving station 6 illustrated in FIG. 1). Typically, a polarization dependent device (PDD), such as a polarization scrambler or an external modulator, is included in optical fiber transmission line 4.
FIG. 3(A) is a graph illustrating an example of a spectrum of the WDM signal light formed by optical multiplexer 18 of transmitting station 2 and before the WDM signal light is transmitted through optical fiber transmission line 4. As illustrated in FIG. 3(A), in this example, the power levels of the signal lights in all the channels are assumed to be equal to each other.
FIG. 3B is a graph illustrating the spectrum of the WDM signal light after being transmitted by transmitting station 2 through optical fiber transmission line 4 and received by, for example, receiver 6 (see FIG. 1). If the spectral density along the vertical axis in FIG. 3(B) is represented by logarithm, a difference in level between signal light power and ASE power corresponds to an optical signal-to-noise ratio (SNR). The optical SNR of signal light after opto-electric conversion (that is, after being converted from an optical signal into an electrical signal) is referred to as the "electric" SNR or simply as the SNR. The optical SNR or electric SNR is an important parameter determining transmission quality of the optical communication system.
As apparent from FIG. 3(B), amplified spontaneous emission (ASE) in each optical amplifier 8 (see FIG. 1) is cumulatively added to the WDM signal light, so that a sharp spectrum of signal light in each channel is superimposed on a relatively gentle spectrum of ASE. As a result, the power levels of the signal lights in all the channels are no longer equal to each other. Thus, the relative power levels of the signal lights have changed, thereby indicating that the "power relativity" of the WDM signal light has changed. Moreover, the relative SNRs of the channels have also changed.
There are several problems in attempting to maintain a constant power relativity of the WDM signal or constant relative SNRs of the channels. For example, referring again to transmitting station 2 illustrated in FIG. 2, the amount of loss of signal light in a specific channel is due, for example, to loss by optical multiplexer 18, connection loss by optical connectors (OC), loss by the polarization dependent device (PDD), and other such losses. As a result, the amount of loss in a specific channel will be different from the amount of loss in a different channel. Further, these losses vary with changes in polarization condition.
Accordingly, even if each optical transmitter 12 (#1 to #m) provides, for example, an automatic level control (ALC) function to maintain the output power level of the channel at a specific level, the power relativity in the WDM signal light output from transmitting station 2 will be varied. Therefore, the optical communication system experiences unstable transmission characteristics.
Moreover, the gain and ASE power of optical amplifier 8 in a specific channel may be different from those in another channel. For example, the gain and ASE power of an optical amplifier for a specific channel depend not only on the power of a signal light in the channel, but also on the power of signal light in other channels. Therefore, when the relative power levels of signal lights in the WDM signal light change in a transmitting station, signal light power and optical SNR in each channel after transmission are varied by optical amplifiers 8.
Accordingly, even if the powers of signal lights in all the channels before transmission are set to equal values (as illustrated, for example, in FIG. 3(A)), the power relativity and optical SNR in the WDM signal light after transmission become different in all the channels (as illustrated, for example, in FIG. 3(B)). As a result, undesireably different transmission characteristics are obtained in the various channels.